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Chapter 17: Darwin’s Theory of Evolution,Brian

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Case Study: Lizards, Legs, and the Diversity of Life,Brian

17 Case Study


Lizards, Legs, and the Diversity of LifeBiologists looking at the living world are amazed at the variety of different living things that share ourplanet. Recognizing and naming all those organisms is tough enough. Figuring out how biologicaldiversity arose has challenged scientists since science began. Some clues to the puzzle can be foundin ancient fossils, while others, depending on where you live, may be staring at you, camouflaged in anearby tree.

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Some biologists try to study life’s “big picture” by examining patterns that encircle the globe andchanges in those patterns that span millions of years. Others focus on smaller subjects and changesthat occur in just a few years.

Professor Jonathan Losos of Harvard University focuses his research on anoles, a group of smalllizards found across the southeastern United States, the Caribbean, Central America, and northernSouth America. There are almost 400 anole species, 150 of which live in Florida and the Caribbean.Why do these lizards come in such a bewildering variety? That’s the question Losos and like-mindedbiologists hope to answer through observation and experimentation. Two patterns seen in anolespecies fascinate biologists.

First pattern: Anole species living on the same island differ in body shape, behavior, and choice ofniche. Species that prefer tree trunks near the ground, for example, have long back legs that enablethem to jump long distances and run quickly. Species that live high in trees have very short legs andstickier toe pads. These species hang on to small twigs and creep slowly to avoid being spotted bypredators.

Second pattern: Anoles living in the same niches on different islands have the same characteristics.BUT … the low-living species on different islands are different species, even though their body shapesand behaviors are similar. What’s more, they are usually not closely related to the low-living species onother islands.

How do these patterns emerge? As part of his efforts to explain those patterns, Losos conductedingenious experiments on small islands along Florida’s Intracoastal Waterway. First, he studied theheight of perches chosen by resident anoles of a native species, Anolis carolinensis. He also measuredtheir legs and toe pads. He then introduced a small population of another species, A. sagrei, whichhad recently crossed to southern Florida from Cuba. What would happen? Would competition causeevolutionary change to occur? If so, how long would that take?

In only a few years, Losos saw evolution taking place before his eyes! The native anoles were movinghigher in the trees than they had been before. Their legs and toe pads were also steadily changing tothe forms found in higher tree-dwelling species! How do these kinds of changes help to explain thediversity of life on Earth?

Throughout this chapter, look for connections to the CASE STUDY to help you answer thesequestions.

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Lesson 17.1: A Voyage of Discovery,Brian

17.1 A Voyage of Discovery


Key Questions

Vocabulary

READING TOOL

As you read the lesson, complete the main idea and detailstable in the Foundations Reading and Study Guide Workbook onyour digital course.

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Charles Darwin sailed on the HMS Beagle.

If you'd met young Charles Darwin, you wouldn't have guessed that his ideas would shake the world.He wasn't a star student. He preferred nature watching and hunting over studying. His fathercomplained, “You care for nothing but shooting, dogs, and rat-catching, and you will be a disgrace toyourself and all your family.” Yet Darwin would come up with what has been called “the single bestidea that anyone has ever had.” That didn't happen because Darwin sailed to exotic places and sawstrange things. Europeans had been exploring and cataloging exotic creatures for years. Darwin didsomething that scientific thinkers described as the true essence of discovery. He saw things that manybefore him had seen, and came up with entirely new ways of thinking about them. And he askedquestions. That's what Darwin did.

Darwin’s Epic JourneyCharles Darwin was born in England on February 12, 1809. Eager to see the world, he got his chancein 1831 when he was invited to join the HMS Beagle on the five-year voyage shown in Figure 17-1.The captain’s job was to map coastlines and harbors. Darwin was added to keep the captain companyduring the long trip. No one knew it, but Darwin’s presence on the Beagle would make this perhapsthe single most important scientific voyage of all time.

The Beagle set sail at a time when scientists in several fields were revolutionizing scientificunderstanding of the natural world. Geologists were suggesting that Earth was ancient and hadchanged over time. Biologists were suggesting that life had also changed, through a process theycalled evolution. But although those other scientists had argued long before Darwin’s voyage thatorganisms could change over time, none of them had offered a scientific description of how that

Figure 17-1 Darwin’s JourneyDarwin’s life’s work involved analyzing the many discoveries and observations he made on thisjourney. He was fortunate to visit a wide variety of habitats, especially the remote and isolatedGalápagos Islands.

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READING CHECK

Darwin’s Epic Journey (cont.),Brian

©April 1, 2020 1:12 pm



Darwin developed a theory of biological evolution that offered a scientific explanation for theunity and diversity of life, by proposing how modern organisms evolved through descent fromcommon ancestors. Recall that a scientific theory is not a guess, a hunch, or a proposal. A scientifictheory is a well-established, scientific explanation of events in the natural world. Scientific theoriescan be used to make predictions about events in the natural world that can be tested by experimentsand observations. Darwin’s work confirmed that the living world is constantly changing. Evolutionarytheory helps us understand events such as the emergence and spread of drug-resistant bacteria andnew strains of influenza. It helps us predict the dangers we will face if human actions drive too manyspecies to extinction. For these reasons, all the biological and biomedical sciences are organizedaround the key ideas that Darwin developed more than 150 years ago.

Summarize How have scientists applied Darwin’s theory of evolution?

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Analyzing Data: Darwin’s Voyage,Brian

17.1 Analyzing Data Standards


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Darwin’s Voyage

1. Using the biome map in Chapter 3 as a model, identify three different biomes that Darwin visitedon his voyage.

2. Find an example of when Darwin visited the same biome on two different continents.

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©April 1, 2020 1:13 pm

Analyze and Interpret Data

1. Identify Patterns Which biome did you identify as the same on two continents? Are similar types ofanimals found on both continents?

2. Infer How was a round-the-world voyage useful to Darwin for developing his theory of evolution?

3. Evaluate Evidence How does evidence from many places around the world, instead of only a singlehabitat or biome, help strengthen Darwin’s theory of evolution?

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Observations from the Voyage,Brian



READING TOOL

Make a table summarizing the observations made by Darwin duringhis travels. What types of variations did he notice globally, locally,and over time?

Observations from the VoyageDarwin was fascinated by the diversity of life he saw during his trip. During a single day in a Brazilianforest he collected 68 species of beetles … and he wasn’t even particularly looking for beetles! Hewas intrigued by how well suited to their local environments plants and animals seemed to be. He wasimpressed by the many ways that different organisms obtained food, protected themselves, andproduced offspring. He was also puzzled by where different species lived—and did not live. He filledhis notebooks with observations.

But Darwin wasn’t content just to describe the diversity he saw. He wanted to explain it in a scientificway. He kept observing, asking questions, and formulating hypotheses. Those hypotheses guided himin making more observations to test those hypotheses. He kept looking for larger patterns into whichhis observations might fit. Over time, Darwin focused on three patterns of diversity: (1) species varyglobally, (2) species vary locally, and (3) species vary over time.

Species Vary Globally Wherever Darwin went, he observed and asked questions. In the grasslands ofSouth America, he found flightless, ground-dwelling birds called rheas, as shown in Figure 17-2.Rheas look and act a lot like ostriches. Yet rheas live only in South America, and ostriches live only inAfrica. Why? Then, when Darwin visited Australia’s grasslands, he found another large flightless bird,the emu. Darwin also noticed that rabbits and other species that lived in the grasslands of Europe didnot live in the grasslands of South America and Australia. And when Darwin visited Australian

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©April 1, 2020 1:13 pm

not live in the grasslands of South America and Australia. And when Darwin visited Australiangrasslands, he saw kangaroos and other animals that are found nowhere else.

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Figure 17-2 Species Vary GloballyOstriches and rheas live in similar habitats on separate continents. Darwin wondered why the twospecies were similar, yet not identical.

Darwin noticed that different, yet ecologically similar, species inhabited separated, butecologically similar, habitats around the globe. What did these patterns of geographic distributionmean? Why did different flightless birds live in South America, Australia, and Africa, but not in theNorthern Hemisphere? Why weren’t there any rabbits in Australian habitats that seemed ideal forthem? For that matter, why didn’t kangaroos live in England?

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CASE STUDY

Species Vary Locally,Brian



Species Vary Locally There were other puzzles, too. Darwin noticed that different, yet related,species often occupied different habitats within a local area. For example, Darwin found twospecies of rheas in South America. One thrived in Argentina’s grasslands, while a smaller species wasadapted to the colder, harsher grass and scrubland to the south.

Darwin observed other examples of local variation in the Galápagos Islands, about 1000 kilometersoff the Pacific coast of South America. These islands are relatively close to one another, but areecologically different. Darwin was impressed and fascinated by the Galápagos, a region that hedescribed as “a little world within itself” and “very remarkable” because it was home to a number ofpeculiar animals, including giant land tortoises such as those shown in Figure 17-3. At first, Darwindidn’t think much about these beasts. In fact, like other travelers, Darwin ate several of them andtossed their remains overboard without studying them closely! Then the islands’ vice-governor toldDarwin that tortoises living on different islands were so different from one another that he could tellwhich island a tortoise came from just by looking at the shape of its shell. Darwin later admitted, “Idid not for some time pay sufficient attention to this statement.”

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Figure 17-3 Galápagos TortoisesThese two tortoise species live on different Galápagos Islands. Like anole lizards, the tortoiseshave traits that vary. Compare and Contrast Describe differences and similarities betweenthese two tortoises.

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Darwin also observed that different islands had different varieties of mockingbirds, all of whichresembled mockingbirds he had seen in South America. He also noticed several types of small brownbirds with beaks of different shapes. He thought some of these birds were wrens, some werewarblers, and some were blackbirds. He didn’t consider them to be especially unusual or important—at first.

Species Vary Over Time In addition to collecting specimens of living species, Darwin also collectedfossils, preserved remains or traces of ancient organisms. Scientists already knew that these remainsformed a fossil record of organisms no longer living, although researchers in Darwin’s day didn’t yetknow how to read and interpret that record.

Darwin noted that the fossil record included many extinct animals that were similar to, yetdifferent from, living species. Among the most striking examples were fossils of extinct giantarmored animals called glyptodonts. You can see in Figure 17-4 that glyptodonts look like giant

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versions of modern armadillos, which live in the same area today. But why had glyptodontsdisappeared? And why did modern armadillos resemble them? Could these animals have had acommon ancestor? Darwin wrote: “This wonderful relationship in the same continent between thedead and the living, will, I do not doubt, hereafter throw more light on the appearance of organicbeings on our Earth, and their disappearance from it, than any other class of facts.”

Figure 17-4 Glyptodont vs. ArmadilloFossils show that ancient organisms differed in important ways from those alive today, yet sharedmany similarities. Ask Questions What questions would you ask about the relationshipbetween the ancient glyptodont and the modern armadillo?

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Putting the Puzzle Together,Brian



Putting the Puzzle Together On the voyage home, Darwin thought about the patterns he had seen.When he returned to London, he visited experts to whom he he sent his specimens for identification.Those experts’ evaluation of Darwin’s specimens set the scientific community abuzz. It turned out thatGalápagos mockingbirds belonged to three separate species found nowhere else on Earth! The littlebrown birds Darwin thought were wrens, warblers, and blackbirds were actually all species of finches!They, too, lived nowhere else, although they all resembled a single common finch species from SouthAmerica. Yet in addition to being different from each other, those island species were different fromthat mainland species too. The same was true of Galápagos tortoises, marine iguanas, and manyisland plants.

Darwin was stunned by these discoveries. The evidence caused him to wonder whether species werereally fixed and unchanging, as people thought back then. Could organisms change over time,through some natural process? Could Galápagos species have evolved from common South Americanancestors? Darwin spent years researching and filling notebooks with ideas about species andevolution.

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Lesson 17.2: Ideas That Influenced Darwin,Brian

17.2 Ideas That Influenced Darwin


Key Questions

Vocabulary

READING TOOL

As you read, identify the science concepts and ideas that influencedDarwin.

Complete the graphic organizer in the Foundations Readingand Study Guide Workbook on your digital course.

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©April 1, 2020 1:14 pm

The Grand Canyon provides evidence of Earth’s long history.

Like all great scientific thinkers, Darwin was profoundly influenced by the work of other scientists. TheBeagle’s voyage came during one of the most exciting periods in the history of science. Geologistsstudying Earth’s structure and history were making new observations and inferences about forces thathave shaped our planet. Naturalists were describing and analyzing connections between organismsand their environments. These and other new ways of thinking about the natural world helped shapeand guide Darwin’s thoughts as he proposed hypotheses and gathered data to test them.

An Ancient, Changing EarthMany Europeans in Darwin’s day thought that Earth was only a few thousand years old, and that ithadn’t changed much during that time. By Darwin’s time, however, a new generation of geologistswere thinking in very different ways about Earth’s history. Geologists James Hutton and Charles Lyellproposed important hypotheses based on the work of other researchers, and on data they uncoveredthemselves. Hutton and Lyell concluded that Earth is extremely old and that the processes thatchanged Earth in the past are the same processes that operate in the present. In 1785, Huttonpresented his hypotheses about how geological processes have shaped Earth. Lyell, who built on thework of Hutton and others, published the first volume of his great work, Principles of Geology, in1830.

Illustration of Valle del Bove near Mt. Etna, Principles of Geology

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Hutton and Geological Change,Brian



Hutton and Geological Change Hutton recognized the connections between geological processesand features, like mountains, valleys, and layers of rock that seemed to be bent or folded. Huttonrealized, for example, that certain kinds of rocks are formed from molten lava. He also realized thatsome other kinds of rocks, like those shown in Figure 17-5, form very slowly, as sediments build upand are squeezed into layers.


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Figure 17-5 Processes That Shape EarthJames Hutton described how geological processes could form and transform rocks, twist them,and lift them to the surface from deep within Earth. Infer How were geologic principlesimportant to Darwin’s work?

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READING CHECK

Hutton also proposed that forces beneath Earth’s surface can push rock layers upward, tilting ortwisting them in the process. Over long periods, those forces can build mountain ranges. Mountains,in turn, can be worn down by rain, wind, heat, and cold. Most of these processes, shown in Figure 17-5, operate very slowly. For these processes to have shaped Earth as we know it, Hutton concludedthat our planet must be much older than a few thousand years. These data and inferences moved himto introduce a concept called deep time—the idea that our planet’s history stretches back over aperiod of time so long that it is difficult for the human mind to imagine.

Lyell’s Principles of Geology Lyell’s great contribution was to argue that laws of nature are constantover time, so scientists must explain past events in terms of processes they can observe in thepresent. This way of thinking, called uniformitarianism, holds that the geological processes we see inaction today are the same processes that shaped Earth millions of years ago. Ancient volcanoesreleased lava and gases, just as volcanoes do now. Ancient rivers slowly dug channels and carvedcanyons, just as they do today. Lyell’s theories required enough time for these changes to occur. LikeHutton, Lyell argued that Earth is much older than a few thousand years. Otherwise, how would ariver have enough time to carve out a valley?

Darwin read Lyell’s books during his Beagle voyage. With Lyell’s ideas fresh in his mind, Darwin hadsome spectacular good fortune. He saw a volcano erupt in Chile, and later learned that anothervolcano 480 miles away had blown its top the same night. Just over a month later, he experienced anearthquake that threw him to the ground and lifted a stretch of rocky shoreline more than 3 metersout of the sea with mussels and other sea animals clinging to it. Still later, when Darwin travelledinland, he observed beds of fossil marine animals in mountains thousands of feet above sea level.

Those experiences amazed Darwin and his companions. But only Darwin turned his observations intoa brilliant scientific insight. Geological events like the earthquake he’d seen, repeated many timesover many years, could push rocks upward a few feet at a time, forming mountains from rocks thathad once been beneath the sea. He could see Lyell’s principle of geological uniformitarianism inaction, with his own eyes! Darwin asked himself, “If Earth can change over time, could life changetoo?”

Infer What evidence or logical reasoning suggested that Earth was muchmore than a few thousand years old?

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READING TOOL

Create a flowchart to show the sequence of events that would havehad to occur for flamingos to gain longer legs, according toLamarck’s ideas.

Lamarck’s Evolutionary HypothesesDarwin wasn’t the first to suggest that species could evolve. The fossil record already provided strongevidence that life had changed over time. Ideas about how species could evolve, however, differed.French naturalist Jean-Baptiste Lamarck proposed two of the first hypotheses about how speciescould change over time. Lamarck suggested that individual organisms could change during theirlifetimes by selectively using or not using various parts of their bodies. He also suggested thatindividuals could pass these acquired traits on to their offspring, enabling species to change overtime. Lamarck published these hypotheses in 1809, the year Darwin was born.

Lamarck’s Ideas Lamarck proposed that all organisms have an inborn urge to become more complexand perfect. As a result, organisms change and acquire features that help them live more successfullyin their environments. He thought that organisms could change the size or shape of their organs byusing their bodies in new ways. According to Lamarck, for example, water birds could have acquiredlong legs because they began to wade in deeper water looking for food. As a bird stretched its legsto stay above the water’s surface, its legs would grow a little longer. Structures of individualorganisms could also shrink if they were not used. If a bird stopped using its wings to fly, for example,its wings would become smaller. Lamarck called traits altered by an individual organism during its lifeacquired characteristics.

Lamarck also suggested that if a bird acquired a trait like longer legs during its lifetime it could pass

Lamarck’s Evolutionary Hypotheses,Brian


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READING CHECK

Lamarck also suggested that if a bird acquired a trait, like longer legs, during its lifetime, it could passthat trait on to its offspring. This principle is called inheritance of acquired characteristics. Over a fewgenerations, species like the flamingo shown in Figure 17-6 would evolve longer legs than theirancestors.

Evaluating Lamarck’s Hypotheses Today, we knowthat Lamarck’s hypotheses are completelyunsupported. Organisms do not have an inborn driveto become perfect. Evolution does not mean that aspecies becomes “better” over time. Also, evolutiondoes not progress in a predetermined direction. Wealso know that traits acquired by individuals duringtheir lifetime (such as loss of a limb), are not inheritedby their offspring.

Still, Lamarck was one of the first naturalists to arguestrongly that species are not fixed. He was among thefirst to propose a scientific description of naturalprocesses he thought could enable species to changeover time. Lamarck also recognized that organisms’adaptations are related to their environments and theway they “make a living.” So, although Lamarck’shypotheses about evolutionary change were wrong,his ideas paved the way for later biologists, includingDarwin.

Draw Conclusions Whatevidence refutes Lamarck’s ideas about passing on acquired traits?

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Figure 17-6 Always Improving?Lamarck thought that organisms such asthese flamingos could improve theirtraits during their lifetimes, and thenpass the improvements to theiroffspring.

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Population Growth,Brian



Population GrowthIn 1798, English economist Thomas Malthus noted that people were being born faster than peoplewere dying, causing overcrowding, as shown in Figure 17-7. Malthus reasoned that if the humanpopulation grew unchecked, there wouldn’t be enough living space and food for everyone. Theforces that work against population growth, Malthus suggested, include war, famine, and disease.

Figure 17-7 So Many PeopleThis engraving shows the crowded conditions in London during Darwin’s lifetime. Cause and Effect According to Malthus, what would happen if the population of Londoncontinued to grow?

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Darwin was thunderstruck when he read Malthus’s work. He realized that if Malthus’s reasoningapplied to people, it applied even more to other organisms. Why? Because many organisms canproduce lots more offspring than humans, and therefore have the potential to produce many moreoffspring than can survive. A maple tree produces thousands of seeds. An oyster can produce millionsof eggs. Evaluating these facts, Darwin realized that if all descendants of even one pair of oysterssurvived, they would eventually overrun Earth. Obviously, this wasn’t happening. In the struggle forexistence, many die and only a few survive and reproduce. This is known in the language ofevolutionary biology as differential reproductive success.

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Lesson 17.3: Darwin’s Theory: Natural Selection,Brian

17.3 Darwin’s Theory: Natural Selection Standards


Key Questions

Vocabulary

READING TOOL

As you read, complete the natural selection concept map inthe Foundations Reading and Study Guide Workbook on yourdigital course.

adaptationfitness

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Variation within a population is necessary for natural selection to occur.

Darwin worked out the main elements of evolution by natural selection soon after reading Malthusand thinking about artificial selection. His scientific friends urged him to publish his brilliantarguments. Darwin did write up a complete draft of his ideas, but he put the work aside and didn'tpublish it for another 20 years. Why? Darwin knew that many scientists, including some of his ownteachers, ridiculed Lamarck's ideas. Darwin's theory was even more radical, so he wanted to gather asmuch evidence as he could to support his ideas before he made them public.

Then, in 1858, Darwin reviewed an essay by Alfred Russel Wallace, a British naturalist working inMalaysia. Wallace’s thoughts about evolution were almost identical to Darwin’s! Not wanting to get“scooped,” Darwin moved forward with his own work. Wallace’s essay was presented together withsome of Darwin’s observations at a scientific meeting in 1858. The next year, Darwin published On theOrigin of Species, which described his ideas in detail. Wallace had the right idea, but Darwin had dataand observations to support his hypotheses.

Evolution by Natural SelectionDarwin’s contribution was to describe a natural process, a scientific mechanism, that could operatelike artificial selection. In On the Origin of Species, he supported his ideas with arguments fromMalthus and Lamarck.

The Struggle for Existence Malthus’s work had convinced Darwin that if all populations have thepotential to produce more offspring than can survive, members of a population will compete for afinite supply of environmental resources. Darwin described this competition as the struggle forexistence. But which individuals would succeed in surviving and reproducing?

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Variation and Adaptation,Brian

17.3 Darwin’s Theory: Natural Selection


Variation and Adaptation Here’s where natural inheritable variation took center stage. Darwinhypothesized that individuals with certain types of inherited variation are better suited, or adapted, tolife in their environment than other individuals. Members of a predatory species that are faster orhave longer claws or sharper teeth can catch more prey. And members of a prey species that arefaster or better camouflaged can avoid being caught.

Any heritable characteristic that increases an organism’s ability to survive and reproduce in itsenvironment is called an adaptation. Adaptations can involve body parts or structures, like a tiger’sclaws. Adaptations can involve colors, like those that make camouflage possible. Some adaptationsare physiological, like the way a plant carries out photosynthesis, or the way an animal hibernates tosurvive harsh winters. Many adaptations also involve behaviors, such as social behavior and avoidanceof predators. All of the animals shown in Figure 17-9 have adaptations that help them feed on insectsthat live in wood.

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Figure 17-9 Many AdaptationsAll of these animals are adapted to feed on insects that tunnel in trees. Long beaks and fingerscan probe for food, as can thorns, when used as tools.

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Survival of the Fittest Darwin, like Lamarck, recognized that there must be a connection betweenthe way an organism “makes a living” and the environment in which it lives. According to Darwin,differences in adaptations affect an individual’s fitness. Fitness describes how well an organism cansurvive and reproduce in its environment.

Individuals with adaptations that are well suited to their environment can survive and reproduce andare said to have high fitness. Individuals with characteristics that are not well suited to theirenvironment either die without reproducing or leave relatively few offspring and are said to have lowfitness. This results in differential reproductive success, or, as some call it, survival of the fittest. Notethat survival here means more than just staying alive. In evolutionary terms, survival means surviving,reproducing, and passing adaptations on to the next generation.

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Natural Selection,Brian



READING TOOL

Before you read the section Natural Selection, look at Figure 17-10.Read the information in the figure, and then write three questionsyou have about it. As you read, answer the questions.

Natural Selection Darwin named his mechanism for evolution natural selection. Natural selection isthe process by which organisms in nature with variations most suited to their local environmentsurvive and leave more offspring. In both artificial and natural selection, only certain individualsproduce offspring. But in natural selection, the environment—not a farmer or animal breeder—influences fitness.

When does natural selection occur? Natural selection occurs in any situation in which moreindividuals are born than can survive (the struggle for existence), natural heritable variation affectsthe ability to survive and reproduce (variation and adaptation), and fitness varies amongindividuals (differential reproductive success). Well-adapted individuals survive and reproduce. Fromgeneration to generation, populations continue to change as they become better adapted, or as theirenvironment changes. Figure 17-10 shows a hypothetical example of natural selection. Note thatnatural selection acts only on inherited traits because those are the only characteristics that parentscan pass on to their offspring.


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READING CHECK

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Natural selection does not make organisms “better.” Adaptations don’t have to be perfect—justgood enough to enable an organism to pass its genes on to the next generation. Natural selectionalso doesn’t move in a fixed direction. If local environmental conditions change, some traits that wereonce adaptive may no longer be useful, and different traits may become adaptive. This can lead to agreat diversity of adaptations in species living in different environments. Importantly, if environmentalconditions change faster than a species can adapt to those changes, the species may become extinct.Since Darwin’s time, we have learned that natural selection is not the only mechanism that leads toevolutionary change. You will learn about other mechanisms in the next chapter.

Compare How is natural selection different from artificial selection?

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Figure 17-10 Natural SelectionForces in the environment, including those caused by humans, cause individuals with lessfavorable traits to pass on fewer or none of their genes. Use Models What selective forces areacting upon the grasshopper population shown here? What is the end result?

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CASE STUDY

Common Ancestry,Brian



Common AncestryNatural selection depends on individuals’ ability to reproduce and leave descendants. Every organismalive is descended from parents who survived and reproduced. Those parents descended from theirparents, and so forth back through time. Just as well-adapted individuals in a species survive andreproduce, well-adapted species survive over time. Darwin proposed that living species aredescended, with changes over time, from common ancestors—an idea called descent withmodification. Over many generations, different sets of changing environmental conditions could leadto adaptations that could cause a single species to split into two or more new species, as with thehoneycreepers in Figure 17-11. For evidence of descent with modification over long periods of time,Darwin pointed to the fossil record. Note that this process requires enough time for descent withmodification to occur! This reminds us how Hutton and Lyell’s work supported Darwin’s theory: Deeptime gave enough time for natural selection to act.

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Figure 17-11 Hawaiian HoneycreepersSimilar to the Galápagos finches, the Hawaiian honeycreepers are a group of diverse birds that aredescended from a common ancestor. Over time, different adaptations evolved for feeding andmating in their respective habitats. Compare How does common ancestry explain Losos’sobservations of anoles?

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Darwin’s idea that natural selection and adaptation can produce new species offered an explanationfor both the unity and diversity of life. Darwin’s sketched his thoughts about descent from commonancestors in the form of a branching tree. His evolutionary tree sketch is shown in Figure 17-12. This“tree-thinking” implies that all organisms are related. Look back in time, and you will find commonancestors shared by tigers, panthers, and cheetahs. Look farther back, and you will find ancestors thatthese felines share with dogs, then horses, and then bats. Farther back still is the common ancestorthat all mammals share with birds, alligators, and fish. Far enough back are the common ancestors ofall living things. According to the principle of common descent, all species—living and extinct—areunited by descent from ancient common ancestors, and exhibit diversity due to natural selectionand adaptation. A single “tree of life” links all living things.

Figure 17-12 The Tree ModelAs shown by the illustration (below), all the finches of the Galápagos Islands descended from acommon ancestor. Darwin was the first to use a tree-like model to represent common descent(right). The very first organisms form the base of the tree, and their descendants form the trunkand branches.

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Lesson 17.3 Review,Brian

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Lesson Review Standards


KEY QUESTIONS1. What three conditions are necessary for natural selection to occur?

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2. How does evolution explain both the unity and the diversity of all living things?

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CRITICAL THINKING3. Construct an Explanation Why is high fitness a function of an organism’s environment? Include

animals from a specific environment, such as polar bears in the arctic tundra or frogs in the tropicalrain forest, as evidence to support your explanation.

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4. CASE STUDY What adaptations might help different anole species survive in their environments?

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Lesson 17.4: Evidence of Evolution,Brian

17.4 Evidence of Evolution Standards


Key Questions

Vocabulary

READING TOOL

As you read, complete the chart in the Foundations Readingand Study Guide Workbook on your digital course to describe theevidence for evolution.

biogeographyhomologous structure

vestigial structureanalogous structure

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Coelacanth resembles fishes that lived millions of years ago.

Darwin's theory depended on assumptions involving many scientific fields. Scientists working ingeology, physics, paleontology, chemistry, and embryology during Darwin's lifetime did not have thetechnology or understanding to test his assumptions. Other fields that are part of evolutionary theorytoday, such as genetics and molecular biology, didn't even exist yet! During the 160 years sinceDarwin published On the Origin of Species, research in all these fields has provided independent testsof hypotheses related to Darwinian theory. All of these tests could have either supported or refutedDarwin's work. Astonishingly, every scientific test has supported Darwin's basic ideas about evolution.

BiogeographyDarwin recognized the importance of patterns in the distribution of life—the subject of the fieldcalled biogeography. Biogeography is the study of where organisms live now and where they andtheir ancestors lived in the past. Patterns in the distribution of fossils and living species, combinedwith information from geology, tell us how modern organisms evolved from their ancestors. Recallthat two observations involving biogeography were important to Darwin’s thinking. First, closelyrelated species can evolve diverse adaptations in different environments. Second, very distantlyrelated species can evolve similar adaptations if they live in similar environments or face similarchallenges in the struggle for existence.

Closely Related but Different To Darwin, the biogeography of Galápagos bird species suggestedthat populations of several island species had all evolved from a single mainland species. Over time,natural selection on different islands selected among individuals with different inherited variations.That caused populations on different islands to evolve into different, but closely related, species.

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Distantly Related but Similar,Brian

17.4 Evidence of Evolution


Distantly Related but Similar In contrast, Darwin noted that ground-dwelling birds in ecologicallysimilar grasslands in Europe, Australia, and Africa resembled one another, although they were notclosely related. Differences in those birds’ body structures provide evidence that they evolved fromdifferent ancestors. But natural selection in similar habitats led to similar adaptations, such as longlegs and feet with toes adapted to running.

The Age of Earth and FossilsTwo potential difficulties for Darwin’s theory involved the age of Earth and gaps in the fossil record.Data collected since Darwin’s time addressed those difficulties and provided dramatic support for anevolutionary view of life.

The Age of Earth Darwin knew that for life to have evolved the way he thought it had, Earth must bevery old. Hutton and Lyell had argued that Earth was a lot older than people used to think, butcouldn’t determine just how old. The discovery of radioactivity and the technique of radioactivedating enabled geologists to establish the age of certain rocks and fossils starting in the earlytwentieth century. If this technique had shown that Earth was young, Darwin’s ideas would have beenrefuted. Instead, radioactive dating indicates that Earth is about 4.5 billion years old—plenty of timefor evolution by natural selection to take place.

Recent Fossil Finds Darwin also struggled with what he called the “imperfection of the geologicalrecord.” Paleontologists in Darwin’s time hadn’t found enough fossils to document the evolution ofmodern species from their ancestors. But that’s changed with the discovery of hundreds of fossils, likethe ancient whale skeleton shown in Figure 17-13. Many recently discovered fossils now showclearly how modern species evolved from extinct ancestors. A fossil series shown on the next pagein Figure 17-14 documents the evolution of whales from ancient land mammals. Other recent fossilfinds connect the dots between dinosaurs and birds, and between fishes and four-legged landanimals. So many intermediate forms have been found that it is often hard to tell where one group

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READING CHECK

begins and another ends. All historical records are incomplete, and the history of life is no exception.The evidence we do have, however, tells an unmistakable story of evolutionary change.

Identify What technology helped to support Hutton and Lyell’s hypothesisthat Earth is much older than many people thought?

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Figure 17-13 Fossil DiscoveriesThis fossil of Basilosaurus, discovered in Egypt, provides evidence of the evolution of whales fromland mammals. It also provides evidence for how Earth has changed over time. This site was oncean ancient sea.

Figure 17-14 Whale EvolutionResearchers have found more than 20 related fossils that document the evolution of whales fromancestors that walked on land. Several reconstructions, based on fossil evidence, are shown here. Infer Which of the animals shown was probably the most recent to live primarily on land?

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ion reads: Modern whales retain reduced pelvic bones and, in some cases, upper and lower limb bones.

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Comparing Anatomy and Development,Brian



READING TOOL

As you read about homologous structures, look for examples in theillustrations shown in Figure 17-14.

Comparing Anatomy and DevelopmentBy Darwin’s time, scientists had noted that the bones in all vertebrate limbs resembled each other toa surprising degree, as shown in Figure 17-15. That resemblance was clear, even though some ofthose limbs were used for crawling, some for climbing, some for running, and others for flying. Whydo the same basic structures appear over and over again with such different purposes?


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Figure 17-15 Homologous StructuresColor-coding is used to show the homologous bones in the forelimbs of select modernvertebrates. These limbs evolved from the front limbs of a common ancestor whose bonesresembled those of an ancient fish. If these animals had no recent common ancestor, they wouldbe unlikely to share so many common structures.

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©April 1, 2020 1:19 pm

Homologous Structures Similar structures, like the bones of vertebrate limbs, that are shared byrelated species and have been inherited from a common ancestor are called homologous structures.Evolutionary theory explains the existence of homologous structures adapted to differentpurposes as the result of descent with modification from a common ancestor. Biologists testwhether structures are homologous by studying anatomical details, the way structures develop inembryos, and the pattern in which they appeared over evolutionary history.

Similarities and differences among homologous structures help determine how recently speciesshared a common ancestor. For example, many bones of reptiles and birds are more similar to oneanother in structure and development than they are to homologous bones of mammals. This showsthat the common ancestor of reptiles and birds lived more recently than the common ancestor ofreptiles, birds, and mammals. So birds are much more closely related to crocodiles than they are tobats, even though both birds and bats have wings! Note that the key to identifying homology iscommon structure and origin during development, not common function. A bird’s wing and a horse’sfront limb (which are homologous structures) have similar structures and development, but differentfunctions. Homology occurs among plants, too. Certain groups of plants share homologous stems,roots, leaves, and flowers.

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Some homologous structures don’t serve importantfunctions. Vestigial structures are inherited fromancestors, but have lost much of their original size andfunction. Recently, researchers in Australia studied aclade of skinks, a type of lizard. This group of skinkshas elongated bodies like snakes. Many of them haveremnants of limbs that they no longer use, as shown inFigure 17-16. Using molecular evidence, theresearchers hypothesized relationships among speciesof this clade and developed a timeline for limb sizereduction. Another example of a vestigial structure isthe hipbones of dolphins. You can see a dolphinhipbone in Figure 17-14. Why do these vestigialstructures persist? One possibility is that they don’taffect an organism’s fitness, so selection does not actto eliminate them.

Analogous Structures Body parts that serve similarfunctions, but do not share structure anddevelopment, are called analogous structures. Thewing of a bee and the wing of a bird are examples ofanalogous structures. Both are used for flight, butthey grow and develop from different embryonic tissues.

Development Researchers noticed long ago that early developmental stages of many vertebrateslook similar. Recent observations make clear that the same groups of embryonic cells develop in thesame order and in similar patterns to produce many homologous tissues and organs. For example, thevery differently shaped limb bones in Figure 17-15 all develop from the same clump of embryonic

Analogous Structures,Brian


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Figure 17-16 Vestigial StructuresLerista lineopunctulata, known as thedotted-line robust slider, belongs to aclade of skinks that are known for theirlimb reduction over the past severalmillion years. Note the small remnant ofa limb resting on the leaf.

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READING CHECK

y y p g p p ycells. Darwin recognized that evolutionary theory offers the most logical explanation for thesesimilarities. Similar patterns of embryological development provide further evidence thatorganisms have descended from a common ancestor.

Darwin could not have anticipated, however, the support for his theory that would come fromevidence he couldn’t see. An incredible amount of evidence regarding homology has been found fromstudies in genetics and molecular biology that investigate genes that control the development ofboth visible and microscopic homologous structures.

Summarize How do homologous structures provide evidence forevolutionary relationships?

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Genetics and Molecular BiologyThe most troublesome “missing information” for Darwin had to do with inheritance. Darwin had noidea how heredity worked, and he was deeply worried that this lack of knowledge might prove fatalto his theory. Today, genetics provides some of the strongest evidence supporting evolutionarytheory. A long series of discoveries, from Mendel to Watson and Crick to genomics, help explain howevolution works. At the molecular level, overwhelming similarities in the genetic code of allorganisms, along with homologous genes and molecules, provide evidence of common descent.Also, we now understand how mutation and gene shuffling during sexual reproduction produce theheritable variation on which natural selection operates.

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Natural Selection,Brian



Natural Selection The Grants’ data showed that there is lots of heritable variation in beak size andshape in finch populations. But is that variation related to differential survival and reproduction? Luckwas on the Grants’ side, because during their study, a severe drought hit the islands. Plants producedfewer seeds, so the number of seeds dropped steadily. Here’s where things got interesting. As thedrought continued, the birds ate the smaller and softer seeds first. So, over time, only the largest,hardest seeds remained. Many birds starved, and the total number of birds on the island fell. But theGrants’ data showed that certain birds were more likely to survive than others. Individuals with thelargest beaks had higher evolutionary fitness. Drought caused the average beak size in this finchpopulation to change dramatically over just a few years. Figure 17-18 shows the trends that theGrants observed during the drought.

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Figure 17-18 Data From the Galápagos

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This amazing study supported two important hypotheses. The Grants documented that naturalselection takes place in wild finch populations frequently, and sometimes rapidly. One of theGrants’ colleagues calculated that it might take no more than 12 to 20 such droughts to change thebirds’ beak size enough to transform one species into another. Also, this work showed the importanceof variation in providing raw material on which natural selection can operate. Variation within aspecies increases a population’s ability to adapt to, and survive, environmental change. If the finchpopulation hadn’t had enough variation in beak size when the drought started, they would not havebeen able to adapt and change.

Evolutionary Theory Evolves Many scientific advances have confirmed and expanded Darwin’shypotheses. Today, evolutionary theory is vital to all biological and biomedical sciences, and is often

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Careers on the Case: Wildlife Photographer,Brian

17 Case Study Wrap-Up


Careers on the CaseWork Toward a SolutionTaking outstanding animal images requires a wide knowledge of biology, especially behavior andecology. Behavior is often best documented using photos and videos.

Wildlife PhotographerThe best wildlife photographers do much more than just stalk their subjects. They need to know whenand where to look for those magic moments of behavior, and how to capture the right images.

Watch this video to learn about other careers in biology.

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17 Study Guide


Lesson Review17.2 Ideas That Influenced DarwinAs Darwin worked out his theory of evolution, he was influenced by other scientists of his time.Hutton and Lyell concluded that Earth is extremely old and that the processes that changed Earth inthe past are the same processes that operate in the present. Darwin wondered, if Earth’s surfacecould change over time, could life also change over time?

Lamarck suggested that organisms could change during their lifetimes by selectively using or notusing various parts of their bodies. He also suggested that individuals could pass these acquired traitson to their offspring, enabling species to change over time. Lamarck’s hypotheses were not correct,but he did recognize that there is a connection between an organism’s environment and bodystructures.

Malthus reasoned that if the human population grew unchecked, there wouldn’t be enough livingspace and food for everyone. Darwin realized this was true for other organisms. If more offspring areborn than can survive, what causes some to survive and others to die?

In artificial selection, nature provides the variations, and humans select those they find useful. Darwinrealized that these natural variations also provide the raw material for evolution.

artificial selection

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Organize Information,Brian

17 Study Guide


Organize InformationThe factors that are required for natural selection to occur are listed in the table. In the right column,describe these factors and how they contribute to Darwin’s theory of evolution by natural selection.

Organize Information,Brian

©A il 1 2020 1 23

Factor Contribution to Natural Selection

Struggle for existence 1.

Variation and adaptation 2.

Differential reproductive success 3.

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Performance-Based Assessment: Evolution in Action: Beak size among Darwin‘s Finches,Brian

Performance-Based AssessmentSCIENCE PROJECT

Standards


Evolution in ActionBeak Size Among Darwin's Finches

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Darwin was just the first scientist to be puzzled and inspired by Galápagos finches. Peter andRosemary Grant have been studying free-living birds there for decades. Their careful tagging andreleasing of individual birds, combined with detailed measurements of beak size, have produced anamazing wealth of data. Those measurements have provided what researchers call “baseline data” onbeak sizes. As you can see from the graphs shown here, beak size varies a lot. That’s important,because beak size determines how efficiently birds can eat seeds of different sizes and levels ofhardness.

When a severe drought hit, plants on the island produced fewer seeds. Hungry birds soon devouredthe smaller, softer seeds they prefer. What was left? Larger seeds that were harder to eat. Whathappened? Only half the tagged birds survived. But which half? Study the graphs. Then use the dataand your knowledge of evolution to answer these questions.

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CHAPTER 17 Assessment


KEY QUESTIONS AND TERMS


8. According to Malthus, what would occur if the human population grew unchecked?

A. evolution of a new speciesB. extinction of humansC. disease, war, or famineD. development of new traits in humans

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9. Which of the following would an animal breeder use to increase the number of cows that give themost milk?

A. overproductionB. genetic isolationC. acquired characteristicsD. artificial selection

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10. Describe an example of selective breeding.

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Lesson 17.3: Darwin’s Theory: Natural Selection,Brian


Standards


KEY QUESTIONS AND TERMS


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©April 1, 2020 1:24 pm

13. The akiapola’au uses its long top bill to probe for insects in trees. This beak is an example of

A. an adaptation.B. fitness.C. an acquired characteristic.D. a variation.

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14. During the process of natural selection, what determines which organisms survive and reproduce?

A. number of adaptationsB. variety of adaptationsC. evolutionary fitnessD. population size

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15. How does natural variation affect evolution?

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16. According to the principle of common descent, what explains the diversity among organismstoday?

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17. Do all adaptations involve body structures? Give an example to support your answer.

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Critical Thinking,Brian


Standards


CRITICAL THINKING

24. Integrate Information Many scientists of Darwin’s time were experts at identifying plant andanimal species. How did Darwin’s work differ from the work of these other experts?

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25. Infer In all animals with backbones, oxygen is carried in blood by a molecule called hemoglobin.What could this physiological similarity indicate about the evolutionary history of vertebrates -(animals with backbones)?

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26. Construct an Explanation How does Darwin’s theory of evolution help to explain the spread ofdrug-resistant bacteria?

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Use the illustration of mice shown below to answer questions 27 to 29.

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27. Interpret Visuals Which of the mice might be better adapted to their environment? Explain youranswer.

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28. Construct an Explanation What event might change the relative fitness of the different mice?Explain how the change in fitness could occur.

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29. Synthesize Information Consider possible changes that could affect the mice. How is thevariation among the mice useful to the mouse population?

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30. Evaluate Models According to Lamarck’s ideas, an animal’s actions could affect inheritance. Anexample is a wading bird that stretches its legs, who then passes the trait of longer legs on to itsoffspring. Cite evidence to evaluate this model of inheritance.

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31. Develop Models Darwin used the concept of artificial selection to model animal breeding, inwhich humans selected the traits that animals pass to offspring. How did Darwin develop thismodel to explain evolution in natural systems?

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32. Use Scientific Reasoning A scientist discovers a new species of vertebrate, or animal with abackbone, in a desert ecosystem. According to Darwin and to modern ideas about evolution,what kinds of traits can the species be expected to display? Include traits at the molecular level.

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Crosscutting Concepts,Brian



CROSSCUTTING CONCEPTS

33. Patterns Describe one of the patterns among species that Darwin identified. How did this patternhelp him develop his ideas about evolution?

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34. Cause and Effect According to Darwin’s ideas, what is the cause of a change in the adaptations ofa species over time? Describe a specific example.

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Math Connections,Brian


Standards


MATH CONNECTIONS

Analyze and Interpret Data

Use the following paragraph and the line graph to answer questions 35 to 37.

In the 1990s, scientists were studying a colony of brown anoles (Anolis sagrei) on a Caribbean island.These brown anoles live in trees and perch on branches. In 1996, a large, predatory lizard speciesarrived on the island, and they began hunting the brown anoles. The graph shows the change in theaverage diameter of the tree branches where the scientists found brown anoles.

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35. Interpret Graphs How did the average branch diameter of the brown anoles’ perches changeduring the time of this study?

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36. Construct an Explanation How might the introduction of the predatory lizard species havecaused the change in perch diameter shown in the graph?

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37. Predict As a result of the introduction of the new predator, what other changes may occur to thebrown anole population over time? Use the concepts of fitness, adaptation, and natural selectionto justify your prediction.

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Use the following paragraph and the chart to answer questions 38 and 39.

Cytochrome c is a small protein that takes part in cellular respiration. The table compares thecytochrome c of various organisms to that of chimpanzees.

MathConnections,Brian

©April 1, 2020 1:26 pm

Organism Number of Amino Acids That Are Different FromChimpanzee Cytochrome c

Dog 10

Moth 24

Penguin 11

Yeast 38

38. Interpret Data How can you interpret the data in the chart to draw a conclusion about theevolutionary relationships among the species?

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Language Arts Connection,Brian


Standards


LANGUAGE ARTS CONNECTION

Write About Science

40. Write Informative Texts Write a paragraph to explain how studies in genetics and molecularbiology provide evidence for the theory of evolution. Include specific examples in yourexplanation.

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41. Write Explanatory Texts Summarize the conditions under which natural selection occurs. Then,describe three lines of evidence that support the theory of evolution by natural selection.

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Read About Science

42. Determine Conclusions What conclusion did Darwin reach about the patterns he observedamong Earth’s living things?

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Chapter 17 End-of-Course Test Practice,Brian

End-of-Course Test Practice


1. A pond has several varieties of a species of frog that all eat the same kind of bug. Most of the frogsare fast-growing varieties that need to eat more bugs each day than the slow-growing variety. Adrought drastically reduces the number of bugs, and the population of the fast-growing frogsdeclines drastically while the population of slow-growing frogs declines only slightly, so that mostof the frogs in the pond are now the slow-growing variety. This is an example of which of thefollowing?

A. mutationB. natural selectionC. biogeographyD. artificial selectionE. acquired characteristics

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2. Thomas Malthus argued that if the human population grew unchecked, then supplies of food andother resources would be exhausted. Which of these ideas about evolution did Darwin adapt fromMalthus’s work?

A. An organism passes many of its traits to its offspring.B. All organisms fill a niche, or a role in their environment.C. Variations within a species are necessary for evolutionary change.D. Organisms are able to produce many more offspring than will survive.E. The environment selects the traits that an organism passes to offspring.

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3. Two species of fish are introduced to the same lake. Over time, only one species survives.According to Darwin’s theory of evolution, what should characterize the surviving species?

A. greater genetic variationB. more abundant food supplyC. a better ability to survive and reproduce in the lakeD. a better ability to withstand seasonal changes in the lakeE. common ancestors with other fish in the lake

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4. Darwin’s theory of evolution offers a scientific explanation for which of the following?

A. how genetic information is inheritedB. changes in species over timeC. human effects on biodiversityD. the age of EarthE. the origin of life

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5. Bernice is preparing a report on evidence for the common ancestry of species. She includes thefigure of forelimbs shown.

The forelimbs are an example of which of the following?

A. analogous structuresB. vestigial structuresC. homologous structuresD. convergent structuresE. mutated structures

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Chapter 17 End-of-Course Test Practice,BrianIf You Have Trouble With…

Question 1 2 3 4 5

See Lesson 17.3 17.2 17.1 17.4 17.4

Performance Expectation HS-LS4-2 HS-LS4-2 HS-LS4-4 HS-LS4-3 HS-LS4-1

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